It looks like any other happy family vacation – a father and his stepson lounging together in a swimming pool.

But six months later, an unimaginable horror would strike.

Retired cop Scott Nager, who was allegedly beheaded by his samurai sword-wielding stepson Zachary Gibian on Feb. 27, was on good enough terms with the teen to enjoy a family holiday in Mexico with him last fall.

Nager, 51, his wife, Laura, and Gibian, 18, were in Cancun with an ex-NYPD pal, Al Guagenti, and his family.

Now the devastated ex-cop is one of many struggling to understand how a once bright and polite teen could attack his stepfather with a sword as he slept on the couch in his Hauppauge, L.I., home.

“The Zach I saw there [in court] was not the Zach I knew – it was like a totally different person,” said Guagenti, who attended Gibian’s arraignment Monday for the murder.

But he said his old pal had started to worry about the teen and suspected Gibian was taking drugs, which made him behave strangely.

Students at Hauppauge HS, where Gibian was a senior, also said the teen used to be polite, smart and very popular, but had recently withdrawn and started dabbling in narcotics.

“He got into popping pills like Vicodin and smoking weed,” said one student, who had been friends with Gibian since elementary school.

Some of Gibian’s friends have speculated that Nager beat his stepson, but Suffolk County police said there was no evidence of violence in the family. A police source said the teen did not mention it in his videotaped confession to the murder.